The Air Force announced Sunday morning that it has successfully landed its unmanned mini shuttle, the X37B, at Kennedy Space Center.

"The #AirForce #37B #OTV4 has returned from orbit and has landed safely @NASAKennedy," the Air Force tweeted.

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This was the fourth flight for the military research program, which is shrouded in secrecy. The shuttle that landed Sunday had been in orbit for more than 700 days, the Air Force said.

The last X-37B mission lasted close to 675 days and ended with a California touchdown.

On social media, Floridians as far away as Tampa reported hearing a sonic boom around 7:45 a.m.

The X-37B looks like a miniature version of a NASA space shuttle. Like the old shuttle, the X-37B launches vertically and lands horizontally, is reusable, and has lots of room for experiments. But no one flies on them; they are operated robotically.